Lots of coil questions here lately - Here is some info on 8in concentric coil from Kellyco

[DonNWF]

I've been interested in the 8in Excelerator Concentric Coil for my Sov/GT since finding out that concentric work really well in low mineral soil, and thats what I have here in Coastal NW Florida. My other coils are Dbl-D's. I use their little 5 in Coil for trash areas but wanted more depth. My standard coil is the 10in Tornado (good coil but masks too much in trash). So last month I bought the 8in SOV Concentric. As soon as I hooked it up to my GT using my standard setup (GT w /meter and S-1 Probe, Disc mode W/iron mask on, 11 Sens, max vol.) I knew something was wrong. I could not get the threshold to lock in except at a max setting (Normally my threshold comes in solid a little below mid-point. I found that I could get a reasonable threshold by running the coil only thru the meter, no probe. I tested all alternate settings except a change in Vol control and nothing worked. I don't hunt without my probe I assumed that I had a bad coil I contacted JW at Kellyco and he immediately arranged for me to get a replacement coil, no problem - no questions. Thanks JW.

As soon as I received my replacement coil and hooked up, it appeared that I still had the same problem. Went hunting with the best threshold I could get to set, but everything seemed over-powered. Marginal tones, etc.. But it WAS working with good separation in a high trash area. Then I accidentally bumped my Vol control off max and immediately noticed a change in threshold. SURPRISE --When I set the Vol control at 3 O'Clock everything settled down and I was able to set a normal threshold at the mid-point. I also found that setting my Sens to the one O'Clock position made everything even better and just brought out the best from the GT. When the Vol control is set at max the threshold goes silent and must be adjusted. I cannot explain this.

Minelab recommends a max vol setting, but does show a startup for both A/M and Disc mode using a 3 O'Clock set. Maybe the Vol control is something like setting a gain control - I don't know.
Maybe Rick (ND) or ART (OH), or other long time SOV users can give me a clue.

But I'm not worried about it. The coil works fine with all my other tools and 10hrs using this coil it has fulfilled all my expectations for it. Excellent tones, its hot, even at a reduced Sens of 1 or 2 O'Clock. Sens can be increased but the tones are not as good. Works great in trash - Pinpoints spot on. Another good tool. I like it. I haven't tried it on the beach yet. Don

 

[Art (NWOH)]

Never saw that happen. Had to read your post 3 times to make sure I understood it.

Going to be up to you to experiment a little more to see how it is going to work.
HH

 

[Rick(ND)]

I really don't know what to think, but have you tried this with both the headphone and the speaker?? Was thinking if this was with the headphones maybe there was a problem there. I tried a proto type of another brand Concentric coil and was very impressed with it and even could turn up the sensitivity even more. I am hopping someone else with the Excellerator concentric coil would say what they are seeing as maybe you got a second bad coil as the Concentric coil I tired was awesome, ran smooth and had great depth to it also with the sensitivity at 10 and volume at max.

 

[ejudasf]

n/t

 

[Rick(ND)]

The one I did a test with is not being made at this time, but maybe at a latter time. I like what I seen and wonder how well it would sell if it was made.

 

[DonNWF]

Did a little testing in A/M and seems hotter than my other coils, but the grnd was damp , so that may not be a normal response.
I did strip the GT down to just the coil and control box and with my standard settings I got typical responses to all targets in my test bed. Even went to 9 and 10 O'Clock Sens (which is usually no problem here anyway) and the tones were excellent. Go figure I'm going to keep the coil and I'm sure it will work for me.

My post was not to knock the coil in anyway. I don't want anyone to assume it isn't any good. I'm fine with it. I just wanted potential users to be aware of the strange operational responses I have seen and maybe someone had a similar experience or suggestion.

My little test with a strpped down SOVGT almost dislocated my shoulder I swung it so high. Maybe GT light is a good thing anyway. HH

 

[Art (NWOH)]

Hey Don.......you say it worked right with only the coil hooked to the detector?

If so then I think I know what the problem is, but I don't think you can do anything about it. Involves high freq parasitic oscillations getting into the electronics. The length of the cable seems to affect this.

Probably a design deficiency........
Tested with detector only, not allowing for a probe or meter connected.

JW at Kellyco needs to know about this.
HH

 

[DonNWF]

Yes - I disconnected everything but the coil so that I started testing from a basic setup. All the weirdness went away.

I just measured the coil cable and it is 6 feet in length. As you know the coil cable goes into the probe control box then a short jumper cable goes from the probe control box into the meter, than 4 feet of cable goes to the GT control box. This many cables are a pain to deal with anyway. That I have Parasitic oscillations wouldn't surprise me. I'm guessing at what that means. If you dont mind I will invite JW to view your post in case he misses this thread. Question - would a shorter coil cable have any effect?

 

[Art (NWOH)]

What JW needs to know ........
Works with coil only. Screws up with accessories installed between the coil and the box. A description of the symptoms needs to be included.

Shorter cables may or may not fix the problem. Could make it worse. I messed up a coil once by shortening the cable. Made the meter read too low...could not calibrate. It's a long story that I will skip.

Parasitics (probable cause).....
Every cable has a particular impedance (resistance, inductance, and capacitance per foot) . There is characteristic impedance that is not length dependent , but determines if cables are matched (VSWR). This equates to a resonant circuit.....resonant at a particular frequency .
Different kinds of cable have different characteristics, with impedance mismatch between them causing power reflections.
Sovs have a pulse (square wave ) output which can shock excite oscillations to occur in particular lengths of cable because of their resonance. The oscillations bounce back and forth from end to end of the cable, and would die out under most conditions........
But if a new pulse hits at the right time interval (in phase with the reflecting pulse) it will strengthen the reflecting pulse (oscillation).
It becomes self sustaining by feeding off the transmit pulse's power.......a parasite.

This high freq oscillation can couple onto the recieve lines, as well as run back up the transmit lines to the control box. The signal has the wrong freq range to be filtered out , so will crawl all over the ckt board and create havoc.

I don't really like my answer, but I have not had a beer yet and don't think too well in this condition.
If I totally botched this up, someone please straighten it out for me.


Clear as mud right?

HH

 

[DonNWF]

Hell of an answer. I have an old rule I follow never to have strong drink until after Retreat (Army term for - the flag comes down and the drinks are pored). I will observe that rule today followed by a double gin while I reread your response. In-depth study may require a refill. When clarity is called for I go for what works for me. As a change of pace the GT goes out tomorrow naked (w/Cons-coil), except for the operator of course. Change is good.
Disc me out

 

[Art (NWOH)]

Let's say that all the concentrics do this........
If I liked them, and didn't mind possibility losing the cost of a coil,.....I might hack a little cable off to see if it helped.
That's a lot of beer money to throw away if it don't work, though.

HH

 

[Art (NWOH)]

I read on one of the forums a while back about putting a clamp on ferrite on the cable. (split core) Don't remember what it was supposed to do.....
May help de-tune the cables and snub out unwanted oscillations.

HH

 

[DonNWF]

Hi Art - I did some reading on these Ferrite things after reading your post. Being electronically challenged I cannot determine what type of Ferrite Core to use both types take care of parasitic ocillations. There is listings for EMI and RFI types. Can you tell me which one to use or maybe both? I think this might be worth a shot. Don

 

[Art (NWOH)]

I was afraid you would ask that.
Don't have any idea which ones may work.
I would think that one made to snuff oscillations in the VHF band would do........Just guessing based on the cable lengths involved. Location on the cable(s) or on the RIGHT cable could be important.

Wish I could remember which forum it was on. Think it was to improve performance, but don't remember for what detector.
Kind of like reading about running the coil cable straight up the lower rod as far as possible to improve performance.

Reminds me.....you do have as much extra cable wrapped up as far above the coil as possible, right? .....keeping as much of the cable away from the transmitting field as possible..

I'm not an engineer, so can't help much more than that.

HH

 

[Art (NWOH)]

Probably the EMI choke........
I looked over some on one site and found that those look more for the lower RF spectrum signals.

I doubt that with the cable lengths involved that an oscillation would be above the VHF band. It might be possible to pick it up on a spectrum analyzer, but not very many of us have them.

Sooooo....maybe a snap on EMI filter made for 50-300 mhz would work.
It's just a guess.

HH

 

[DonNWF]

n/t

 

[JW]

also the coil was designed and tested w/o any other accessories attached.
JW

 

[Art (NWOH)]

Thanx JW. I know that the long coil and meter cables are for hip mounting.

There seems to be a glitch when meters and probes are added.
Never saw a problem with the 12.5 and 14" Excelerators.

HH